1. Field of the Invention
The present invention relates to an apparatus and a method for detecting a toner density, more particularly, to an apparatus and a method for detecting a toner density in a liquid developer obtained by mixing charged toner particles in a carrier solution.
2. Description of the Related Art
There is a related art image forming apparatus that has adopted the wet type developing method for feeding a developer onto the peripheral surface of a photosensitive drum that is bearing a static latent image thereby to visualize the static latent image. In this type image forming apparatus, the image quality is affected by the toner density and the toner charging state in the liquid developer, which are varied according to the consumption of toner particles. Consequently, it is indispensable to know the toner density and the toner charging state in the liquid developer and keep the toner particles in a stable state for such an image forming apparatus. A technology for detecting the toner density and the toner charging state such way is thus very important.
There have been made various propositions concerning the above technology so far. For example, the official report of Unexamined Published Japanese Patent Application Laid-Open No. 3-295453 (Issued on Dec. 26, 1991) discloses an evaluation method for a liquid developer used for electrophotography, which detects a charging volume of toner particles by measuring the conductance of an object liquid developer. And, the official report of Unexamined Published Japanese Patent Application Laid-Open No. 63-303380 (Issued on Dec. 9, 1988) discloses a method for controlling a toner density according to a toner feed signal issued corresponding to the inter-electrode conductance of a pair of electrodes disposed in a liquid developer.
Furthermore, the official report of Unexamined Published Japanese Patent Application Laid-Open No. 56-39570 (Issued on Apr. 15, 1981) discloses a method for controlling an image density by measuring the transmittance of a liquid developer using a photosensor and corresponding this measured value to the resistance value of a carrier solution measured using an electrode. And, the official report of Unexamined Published Japanese Patent Application Laid-Oped No. 63-5751 (Issued on Feb. 4, 1988) discloses an apparatus for detecting an effective toner density by combining an electrical field and transmitted light detection. In addition, the official report of Unexamined Published Japanese Patent Application Laid-Open No. 1-179064 (Issued on Jul. 17, 1989) discloses a method for controlling a toner density by measuring the conductance of each color liquid developer used for a common developing apparatus using the same method as that disclosed in the official report of Unexamined Published Japanese Patent Application Laid-Open No. 63-303383.
A liquid developer used for electrophotography is obtained by mixing charged toner particles in a carrier solution. In addition to such charged toner particles, the liquid developer also includes a counter ion and an excess ion generated by a charging control agent (CCA). The counter ion has a reverse polarity of that of charged toner particles. The excess ion usually has the same polarity as that of charged toner particles.
In the case of the evaluation method disclosed in the official report of Unexamined Published Japanese Patent Application Laid-Open No. 3-295453, the electric charge of toner particles is measured by a pair of electrodes thereby to measure the conductance of a liquid developer as a whole; however, a measured charge includes the electric charges of the counter ion and the excess ion. This is why it is difficult to measure only the electric charge of toner particles in a liquid developer. In the case of the toner density controlling method disclosed in the official report of Unexamined Published Japanese Patent Application Laid-Open No. 63-303380, no toner density can be measured. Because, the measured conductance is greatly effected by the counter ion and the excess ion.
In the related art disclosed in each of the official reports of Unexamined Published Japanese Patent Application Laid-Open No. 63-5751 and No. 56-39570, a change of light transmittance is used effectively to detect changes of toner density, thereby the problem arisen in the related art electrical measurement is solved. In this case, however, the following problem will arise from a measurement of light transmittance. In other words, when measuring a light transmittance, it is indispensable to take some measures including those for dipping a pair of photosensors, each being composed of a light emitting element and a light receiving element, in a liquid developer or for disposing a photosensor portion provided with a transparent measuring window that narrows the passage gap through which the liquid developer passes. The method for dipping a pair of photosensors in a liquid developer is confronted with a wiring problem, as well as a contamination problem to be caused by liquid developer particles stuck on the light emitting and/or the light receiving surface. This is why it is difficult to put the method on practical use. The method for measuring using a measuring window will also be confronted with stains on the measuring window, as well as mixing of bubbles and/or foreign matters into the measuring gap, resulting in degradation of the reliability of measurement results. In addition, in the case of the sensing method that uses a transmitted light, it is difficult to obtain stable measurement results, since the transmission level differs between when respective color liquid developers are used and when a black liquid developer is used, as well as among types of liquid developers in use.
The technology disclosed in the official report of Unexamined Published Japanese Patent Application Laid-Open No. 1-179064 measures a conductance by disposing a pair of electrodes just like in the technology disclosed in the official report of Unexamined Published Japanese Patent Application Laid-Open No. 63-303380. Thus, it is impossible to measure the density of charged toner particles.